1. Field of the Invention
The present invention relates generally to defect detection methods and apparatus, and more particularly to a method and apparatus for automatically detecting defects on semiconductor wafers, photomask reticles, flat panel TV screens, and other devices having repeating arrays or features forming a part thereof.
2. Discussion of the Prior Art
Previously, automatic inspection of defects on wafers and similar objects or surfaces having repeating patterns has been achieved by either comparing one die on the wafer to another die positioned on the same wafer, or by comparing the die to a reference die or to a data base. However, there are two major factors that limit the sensitivity of such defect detection processes:
1. The images of the two comparison dice are not digitized in exactly the same position relative to the image sensor. This is a result of a combination of factors including stage positioning error, spacing error, etc. Although, the offset between the two images being compared can be easily reduced to less than half of a pixel by moving the two images relative to each other within image memory, it can be shown that the residual alignment error, which can be as large as 0.5 pixel, causes artificial differences in the two comparison images and can be mistaken for a defect. To reduce false alarms due to this alignment error, defect detection sensitivity has to be reduced. On the other hand, to increase defect detection sensitivity without increasing the false alarm rate, one has to reduce the alignment error.
2. The two comparison images can have differences caused by normal process variations even though there is no defect. Semiconductor processes, like most processes, have finite tolerances. Normal process variations refer to variations that are within acceptable limits of the process. These can result in variations in line width, variations in reflectivity or contrast of the image, and variations in layer-to-layer alignment from one die to another. For example, variations of line width can be due to exposure variations of the stepper; variations in reflectivity can be due to film thickness variations in the wafer, and layer-to-layer alignment variation can be due to misalignment of the stepper or aligner. These process variations usually occur across wafers from one die to another. However, the observation has been made that within a localized area of a die, such as within one field of view of the sensor, the process remains constant and no significant variations exist.